The present invention relates generally to methods and apparatus for removing entrained gas bubbles from a liquid and; more particularly, to methods and apparatus for deaerating the liquid in stages to prevent downstream operation manufacturing defects that can occur as a result of inadequate bubble removal.
There are a variety of emulsions, suspensions, pastes, and high viscosity liquids used in the manufacture of or which become part of a variety of products in the chemical, pharmaceutical, food product, and photographic industries. These emulsions, suspensions, pastes, and high viscosity liquids often contain entrained air or gases present in the form of small bubbles. Often this air or gas, particularly in the case of entrained bubbles, is detrimental to the final product being produced. For example, in the case of photographic emulsions containing bubbles, the quality of the films or photographic papers produced is greatly impaired, giving rise to coated defects making the photographic materials unusable.
It is known to remove gas bubbles from solutions, emulsions, and other liquid compositions by exposing them to an imposed ultrasonic energy field. In such an energy field, large entrained gas bubbles are caused to coalesce and rise into a gas trap. Small bubbles may be collapsed and the gas driven into solution, depending upon the size of the bubble and the degree of gas saturation of the liquid composition. Apparatus for debubbling generally includes a metal vessel or tube containing a metal horn extending through an end wall of the vessel; one or more crystal transducers resonantly responsive to an imposed ultrasonic RF signal and bonded and/or bolted to the external end of the horn; and an RF signal generator of the proper frequency. Typically, debubbling apparatus in the art of preparing photographic emulsions are operated at ultrasonic frequencies between 25 kHz and 40 kHz.
An apparatus which is typically used in the photographic industry for de-bubbling photographic emulsions is an end cap round ultrasonic bubble eliminator, typically referred to as an ECR. The ECR includes a transducer horn assembly (hereinafter referred to as a xe2x80x9cTHAxe2x80x9d) which is an electromechanical device that converts electrical vibration to mechanical vibration. One particular ECR, with its component THA, is taught in U.S. Pat. No. 5,373,212 to Beau, hereby incorporated herein by reference. In the operation of an ECR, an alternating voltage is applied to a ceramic disc of the THA, which, as a result, generates mechanical vibration. This mechanical vibration assists in the debubbling of the photographic emulsions flowing through the ECR. Beau teaches a debubbling device wherein an ultrasonic power supply regulates power output to a predetermined constant level. The output of the generator is automatically adjusted to maintain a nominal power level, for example, 40 watts, in the face of changing load conditions that would otherwise cause the power to change in the absence of this feature. This is referred to in the prior art as a xe2x80x9cconstant powerxe2x80x9d setting of the generator.
U.S. Pat. No. 5,853,456 to Bryan et al, hereby incorporated herein by reference, discloses a debubbling device suitable for use in debubbling photographic compositions.
The use of ultrasonics in the debubbling or deaeration of liquids is widespread. For example, U.S. Pat. No. 3,239,998 to Carter et al. uses ultrasonics to debubble multiple liquids simultaneously, while U.S. Pat. No. 5,834,625 to Kraus Jr. et al. describes removing air from a discrete sample of liquid using ultrasonics. Other, more simplistic but similar techniques employing a vessel and ultrasonic transducer(s) propose operation under a slight vacuum pressure, allowing trapped gas to be removed from a single solution. Such techniques are taught in U.S. Pat. No. 3,904,392 to Van Ingen et al., U.S. Pat. No. 4,070,167 to Barbee et al., and U.S. Pat. No. 5,372,634 to Monahan.
The vacuum technique, while apparently quite popular, does not appear to assist greatly in bubble removal, and its effectiveness is significantly reduced when dealing with more viscous solutions.
U.S. Pat. No. 4,070,167 to Barbee et al. describes an apparatus with a single ultrasonic transducer placed in the vertical position beneath a horizontal tubular vessel. The apparatus has a fairly complicated recycle stage which includes a further compartment with ultrasonics, typically operated under positive pressure. This setup is quite cumbersome, and there are inherent difficulties both in operation and cleanability of the apparatus in such an arrangement.
In devices which use ultrasonics for debubbling, the acoustic forces emanating from the transducers aid in the separation of gaseous bubbles from solution, as they assist the upward buoyancy force in opposing the downward drag force of the bubbles in solution. This phenomenon has been commonly used in the debubbling of flowing solutions (e.g. U.S. Pat. No. 3,904,392 to Van Ingen et al., U.S. Pat. No. 4,070,167 to Barbee et al., U.S. Pat. No. 5,373,212 to Beau, U.S. Pat. No. 4,398,925 to Trinh et al.).
Because the removal of bubbles from flowing liquids can be critical to the quality of the products made with such liquids and the speed at which such products can be made, increasing the effectiveness of a bubble elimination device is always desirable. Bubbles in solution (in the form of entrained air) are a reality of the modem high-speed methods to coat complex photographic films and papers. If not removed prior to coating, bubbles are a major source of machine down time and coated waste. A single bubble, 30 xcexcm or larger, can cause a coated defect and should be avoided.
Bubbles are introduced into coating solutions, particularly photographic coating solutions in a variety of ways. Bubbles may be introduced directly into the many components used in the various stages of preparation (dissolved, entrained or in voids) of the coating solution before it is delivered to the coating apparatus. Bubbles may also be introduced as result of the mixing process used to create the coating solution itself. Further, bubbles may result from dissolved gases in the coating solution that are released due to changes in pressure and/or temperature. Also, in the preparation of photographic coating solutions, as components are xe2x80x9cmeltedxe2x80x9d from the solid to liquid state, dissolved gases may be released therefrom. As a result, bubbles must be effectively dealt with for an efficient coating process.
The manufacture of complex film and paper photographic products requires the assembly of hundreds of components, high shear agitation, multiple phase changes and many pressure and temperature gradients. All of these factors result in an ideal situation for the formation and maintenance of bubbles entrained in the coating solution. As a result, just prior to coating, the bubbles must be removed. Typically, efforts to effectively remove bubbles from the coating solution have concentrated on enlarging the capacity of the existing deaeration devices, on increasing the time or number of purges prior to coating operation start-up which results, most particularly, in increasing liquid waste.
It is therefore an object of the present invention to provide a staged or graduated method of bubble elimination that effectively removes bubbles from the coating solution.
It is a further object of the present invention to provide a staged or graduated method of bubble elimination that obviates generation of excessive coating solution waste.
Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by removing bubbles from solution by performing primary deaeration of the coating solution in two stages. The first stage of primary deaeration is performed in a kettle or open tank to remove large bubbles (200-300+ microns) and high levels of entrained air (0.1 to 1 percent by volume). This can be accomplished by holding the solution at coating temperature for an hour or so in an open tank or kettle with minimal agitation. Mixer speeds are optimized with level and hold time. The large bubbles (500+ microns) rapidly rise out of solution and vent to atmosphere. With slow agitation, the surface of the solution is turned over and bubble removal is enhanced. The second stage of primary deaeration is accomplished in a smaller tank called a Bubble Elimination Tube (BET). The BET is typically a 6 or 8 inch diameter horizontal cylinder that is 60% filled with the coating solution. Coating solution enters one end of the tube and exits at the bottom of the opposite end of the tube after the solution has been xe2x80x9ctreatedxe2x80x9d for bubbles. The treatment may consist of simple buoyancy, or the coating solution may be treated with one or two ultrasonic horns to aid in driving the bubbles to the liquid/air interface within the tube. The treatment depends on the amount of air in the coating solution and the flow rate and viscosity of the coating solution. As the solution exits the BET, entrained air is typically less than 0.05 percent by volume and bubbles larger than 200 xcexcm have been removed. Optimization of this portion of the process may include adjusting the volume of coating solution in the BET, the power levels at which the ultrasonic horns are operated, BET tank design, or the number of ultrasonic horns in the BET. After second stage primary deaeration is completed in the BET, the coating solution exiting the BET is pumped under pressure into an ECR. The ECR is generally a vertical 3-inch diameter cylinder. Solution enters the top and passes past another ultrasonic horn before exiting the bottom of the ECR. Two processes occur in the ECR. First, the horn forces bubbles to the top of the housing or cylinder. Second, bubbles are dissolved into solution under pressure and effectively removed from the coating solution. Since the bubbles are very small and few in number at this point, they typically stay dissolved in solution through the coating process. Dissolved gases are not an issue in generating coating defects. Coating solution exiting the ECR has negligible entrained air and contains bubbles that are effectively too small (less than 30 microns) to measure.
By using the various devices in an integrated and optimum method, overall deaeration is enhanced without increases in liquid waste or capital cost.